This invention is generally in the field of methods for treatment of fungal infections in plants, especially rust infections of wheat.
Rusts are pathogenic parasitic fungi which infect wheat, barley, oats, beans, corn, sorghum, and other plants. Each rust is generally specific to its host and the location on the plant where infection occurs. Stem rust (Puccinia graminis f. sp. tritici) is a fungus which principally infects the leaf sheath of wheat plants. Leaf rust (Puccinia recondita f. sp. tritici) infects wheat plants through the stomates. Stripe rust (Puccinia striiformis) is similar to leaf rust but differs in that infections appear systemic due to colonization patterns on wheat leaves.
As shown in FIGS. 2 and 3, scanning electron photomicrographs courtesy University of Minnesota SEM lab, rust spores germinate on the waxy surface of the plant, forming germ tubes which migrate laterally across the surface to the stoma where an appressorium is formed. A structure known as an infection peg grows downward through the stoma from the appressoria following chromosomal and protein changes within the appressoria. The peg forms a substomatal vesicle from which infection hyphae ramify inside of the leaf. Continued internal development leads to formation of subcuticular uredia which produce reinfecting urediniospores which are wind-disseminated after uredia rupture the epidermis.
Rust is a good parasite in the sense that it does not kill its host, but reduces yield by stealing nutrients from its host. The plant can be simultaneously infected with other parasites including smuts and other fungi. Powdery mildew, (Erysiphe graminis, also germinates, forms a germ tube, and rapidly develops an appressorium from which the peg is capable of directly penetrating the cuticle of the leaf, in contrast to the rust fungi which must penetrate the leaf through the stoma. Powdery mildew possesses a cutinase enabling it to effect direct penetration.
In 1986 and 1987, approximately 126,000,000 bushels of wheat in the United States were lost to three rusts, stem rust, leaf rust, and stripe rust, an economic loss of greater than $378 million dollars. Each year, major losses occur in some of the nation's wheat-providing states.
Methods presently in use to combat rust infections include 1) use of rust resistant cultivars, 2) topical application of fungicides, and 3) cultural practices. Unfortunately, due to the relatively high rate of mutation of the rust organism, completely new cultivars of wheat are needed every seven years. Fungicides, while effective, are expensive and must be applied as a preventative, even if it is not certain that the plants will be infected. Many of the compounds previously in use have been withdrawn by the EPA. Compounds which are now utilized are more easily degraded and therefore less harmful to the environment but are more acutely toxic to humans. Thus, fungicide applications are even less desirable than before.
The disadvantages and lack of success of these methods are apparent when one considers the huge economic losses which occur each year.
It is therefore an object of the present invention to provide methods and compounds for inhibiting or preventing rust infections.
It is another object of the present invention to provide methods and compounds which are safe, effective, and relatively inexpensive to use.
It is a further object of the present invention to provide methods to reduce rust infection which can be used alone or in combination to increase effectiveness and decrease the probability of developing resistance to the compounds.
It is a still further object of the present invention to provide methods and compounds which can be used topically, after infection occurs, to lessen reinfection.